1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus which suffers little from degradation in image quality due to image artifacts caused by unwanted signals other than magnetic resonance signals.
2. Description of the Related Art
As such an image artifact there is known what is referred to as a zipper artifact which, in a spin-echo imaging pulse sequence, for example, is caused by a free induction decay (FID) signal caused by a 180.degree. pulse mixed in a spin-echo signal (hereinafter referred simply to as an echo signal) obtained as a desired magnetic resonance signal.
FIG. 1 is a schematic representation of the manner in which an FID signal is mixed in an echo signal during its sampling period. In a spin-echo imaging pulse sequence, insufficient selective excitation of a rectangular region by a 180.degree. pulse causes such a leakage component of the FID signal as shown in FIG. 1. The leakage component, combined with the echo signal, produces an artifact on a MR (magnetic resonance) image.
The principle of the production of the image artifact is illustrated in FIG. 2. On the Fourier space (k-space) an FID signal has several frequency components in the readout direction R but only a direct-current component in the phase-encoding direction E. Thus, on an image obtained through two-dimensional Fourier-transform of an echo signal, an artifact Z will be produced at the center (in the neighborhood of the zero frequency in the encoding direction) of the image in such a shape as shown. This type of artifact is variously called "zipper artifact" on account of its shape, or "flow-in artifact". In this specification the artifact is called the zipper artifact.
For the purpose of explaining the cause of the production of the zipper artifact, suppose here that the zipper artifact is represented by Z, and a mixed-in signal (a flow-in signal; here an FID signal) which causes the artifact is represented by fz.
The following methods are known which prevent the production of a zipper artifact.
(1) A method by which a gradient magnetic field called a spoiler is applied after a 180.degree. pulse, thereby reducing an FID signal fz.
This method suffers from drawbacks that the imaging time becomes long and a pulse sequence used becomes complicated because the application of an additional gradient field is needed.
(2) A method by which, in the case of a pulse sequence in which the same echo signal is acquired several times for averaging, the relative direction of an echo signal f and an FID signal fz is reversed each time data is acquired, and their mean values are calculated, thereby canceling the FID signal fz out. This method cannot be used with a pulse sequence in which the number of data acquisitions per encoding step is one.
In order to carry out the method (2), it is only required that the phases of RF pulses be changed with each data acquisition as shown in FIGS. 3A and 3B. FIG. 3A illustrates a case where the phase of each of the 90.degree. and 180.degree. RF pulses in the spherical coordinate system is coincident with x' (x' indicates the x axis in the rotating coordinate system). In this case, an acquired signal Fa will be represented by f-fz with the echo signal f taken as a reference. FIG. 3B illustrates a case where the phase of a 90.degree. pulse is coincident with x', and the phase of a 180.degree. pulse is coincident with -x', in which case an acquired signal Fb will be represented by f+fz. As a result, the FID signal fz can be canceled out by obtaining the arithmetic mean of the acquired signals (=(1/2).times.(Fa+Fb)=f) with the phase of the 90.degree. pulse kept at x' and the phase of the 180.degree. pulse alternated between x' and -x' with each data acquisition as shown in FIGS. 3A and 3B.
According to method (2), the FID signal fz alternates between being in the opposite direction (FIG. 3A) to and being in the same direction (FIG. 3B) as the echo signal f. With the echo signal f taken as a reference, the component of the FID signal fz in the encoding direction becomes the highest-frequency component. This is illustrated in FIG. 4.
In this way, by changing the phases of the RF pulses with each data acquisition (i.e., with each encoding step) so that the direction of the FID signal fz is reversed, the position where the zipper artifact is produced can be shifted from the center of an MR image to its peripheral part (the neighborhood of the Nyquist frequency (highest frequency) in the encoding direction). For this reason, the artifact, which will not disappear from the image, is produced in a peripheral part of the image that is little important for observation, which allows the influence of the artifact on the image to be disregarded practically. In this case, the zipper artifact Za is produced in a position at a distance of L/2, corresponding to the Nyquist frequency defined by a phase encoding quantity, from the center of gradient magnetic fields, as shown in FIG. 5A.
At this point, suppose that parameters in the pulse sequence are defined as shown in FIG. 6. Then, L is given by: EQU T.times..DELTA.GE.times.L=1 (1)
where
T: length of time that a phase-encoding gradient magnetic field is applied in the encoding direction, seconds PA1 .DELTA.GE: unit step in the encoding direction, Hz/cm PA1 L: image field in the encoding direction, centimeters
Consider now a case where it is desired to obtain an image with a region of interest of a patient as its center when the region of interest is at a distance from the center of the magnetic field, i.e., an off-center image. In this case, however, a problem will arise in that the zipper artifact Za shifted to the peripheral part appears in the neighborhood of the image center as shown in FIG. 5B as a result of shifting of the field center. The original zipper artifact Z that makes no shift to the peripheral part of the image will also be produced in a position in the image that is unfavorable for observation.
The above description was made in connection with zipper artifacts in two-dimensional space. The same problems will arise in the case of artifacts in three-dimensional space.